There are a wide variety of internal combustion engines in common usage today. Most internal combustion engines utilize reciprocating pistons with two or four-stroke working cycles and operate at efficiencies that are well below their theoretical peak efficiency. One of the reasons that the efficiency of such engines is so low is that the engine must be able to operate under a wide variety of different loads. Accordingly, the amount of air and/or fuel that is delivered into each cylinder typically varies depending upon the desired torque or power output. It is well understood that the cylinders are more efficient when they are operated under specific conditions that permit full or near-full compression and optimal fuel injection levels that are tailored to the cylinder size and operating conditions. Generally, the best thermodynamic efficiency of an engine is found when the most air is introduced into the cylinders. However, in engines that control the power output by using a throttle to regulate the flow of air into the cylinders (e.g., Otto cycle engines used in many passenger cars), operating at a substantially unthrottled position would typically result in the delivery of more power (and often far more power) than desired or appropriate.
There are a number of reasons that internal combustion engines do not operate as efficiently at partial throttle. One of the most significant factors is that less air is provided to the cylinder at partial throttle than at full throttle which reduces the effective compression of the cylinder, which in turn reduces the thermodynamic efficiency of the cylinder. Another very significant factor is that operating at partial throttle requires more energy to be expended to pump air into and out of the cylinders than is required when the cylinder is operating at full throttle—these losses are frequently referred to as pumping losses.
Over the years there have been a wide variety of efforts made to improve the thermodynamic efficiency of internal combustion engines. One approach that has gained popularity is to vary the displacement of the engine. Most commercially available variable displacement engines effectively “shut down” some of the cylinders during certain low-load operating conditions. When a cylinder is “shut down”, its piston still reciprocates, however neither air nor fuel is delivered to the cylinder so the piston does not deliver any power during its power stroke. Since the cylinders that are shut down don't deliver any power, the proportionate load on the remaining cylinders is increased, thereby allowing the remaining cylinders to operate at an improved thermodynamic efficiency. The improved thermodynamic efficiency results in improved fuel efficiency. Although the remaining cylinders tend to operate at improved efficiency, conventional variable displacement engines have a number of drawbacks that limit their overall efficiency. One drawback of most commercially available variable displacement engines is that they tend to kick out of the variable displacement mode very quickly when changes are made to the desired operational state of the engine. For example, many commercially available automotive variable displacement engines appear to kick out of the variable displacement operating mode and into a “conventional”, all cylinder operational mode any time the driver requests non-trivial additional power by further depressing the accelerator pedal. In many circumstances this results in the engine switching out of the fuel saving variable displacement mode, even though the engine is theoretically perfectly capable of delivering the desired power using only the reduced number of cylinders that were being used in the variable displacement mode. It is believed that the reason that such variable displacement engines kick out of the variable displacement mode so quickly is due to the perceived difficulty of controlling the engine to provide substantially the same response regardless of how many cylinders are being used at any given time.
More generally, engine control approaches that vary the effective displacement of an engine by skipping the delivery of fuel to certain cylinders are often referred to as “skip fire” control of an engine. In conventional skip fire control, fuel is not delivered to selected cylinders based on some designated control algorithm. The variable displacement engines that effectively shut down cylinders that are described above are essentially a class of skip fire engines. Over the years, a number of skip fire engine control arrangements have been proposed, however, most still contemplate throttling the engine or modulating the amount of fuel delivered to the cylinders in order to control the engine's power output.
As suggested above, most commercially available variable displacement engines shut down specific cylinders to vary the displacement in discrete steps. Other approaches have also been proposed for varying the displacement of an engine to facilitate improved thermodynamic efficiency. For example, some designs contemplate varying the effective size of the cylinders to vary the engine's displacement. Although such designs can improve thermodynamic and fuel efficiencies, existing variable cylinder size designs tend to be relatively complicated and expensive to produce, making them impractical for widespread use in commercial vehicles.
U.S. Pat. No. 4,509,488 proposes another approach for varying the displacement of an internal combustion engine. The '488 patent proposes operating an engine in an unthrottled manner that skips working cycles of the engine cylinders in an approximately uniform distribution that is varied in dependence on the load. A fixed amount of fuel is fed to the non-skipped cylinders such that the operating cylinders can work at near their optimum efficiency, increasing the overall operating efficiency of the engine. However, the approach described in the '488 patent never achieved commercial success. It is suspected that this was partly due to the fact that although the distribution of the skipped working strokes varied based on the load, a discrete number of different firing patterns were contemplated so the power outputted by the engine would not regularly match the desired load precisely, which would be problematic from a control and user standpoint. In some embodiments, the firing patterns were fixed—which inherently has the risk of introducing resonant vibrations into the engine crankshaft. The '488 patent recognized this risk and proposed a second embodiment that utilized a random distribution of the actual cylinder firings to reduce the probability of resonant vibrations. However, this approach has the disadvantage of introducing bigger variations in drive energy. The '488 patent appears to have recognized that problem and proposed the use of a more robust flywheel than normal to compensate for the resultant fluctuations in drive energy. In short, it appears that the approach proposed by the '488 patent was not able to control the engine operation well enough to attain commercial success.
Although existing variable displacement engines work well in many applications, there are continuing efforts to further improve the thermodynamic efficiency of internal combustion engines.